Mosh: the mobile shell

Mosh is a remote terminal application that supports intermittent
connectivity, allows roaming, and provides speculative local echo
and line editing of user keystrokes.

It aims to support the typical interactive uses of SSH, plus:

Mosh keeps the session alive if the client goes to sleep and
wakes up later, or temporarily loses its Internet connection.

Mosh allows the client and server to "roam" and change IP
addresses, while keeping the connection alive. Unlike SSH, Mosh
can be used while switching between Wi-Fi networks or from Wi-Fi
to cellular data to wired Ethernet.

The Mosh client runs a predictive model of the server's behavior
in the background and tries to guess intelligently how each
keystroke will affect the screen state. When it is confident in
its predictions, it will show them to the user while waiting for
confirmation from the server. Most typing and uses of the left-
and right-arrow keys can be echoed immediately.

As a result, Mosh is usable on high-latency links, e.g. on a
cellular data connection or spotty Wi-Fi. In distinction from
previous attempts at local echo modes in other protocols, Mosh
works properly with full-screen applications such as emacs, vi,
alpine, and irssi, and automatically recovers from occasional
prediction errors within an RTT. On high-latency links, Mosh
underlines its predictions while they are outstanding and removes
the underline when they are confirmed by the server.

Mosh does not support X forwarding or the non-interactive uses of SSH,
including port forwarding.

Other features

Mosh adjusts its frame rate so as not to fill up network queues
on slow links, so "Control-C" always works within an RTT to halt
a runaway process.

Mosh warns the user when it has not heard from the server
in a while.

Mosh supports lossy links that lose a significant fraction
of their packets.

Mosh handles some Unicode edge cases better than SSH and existing
terminal emulators by themselves, but requires a UTF-8
environment to run.

Mosh leverages SSH to set up the connection and authenticate
users. Mosh does not contain any privileged (root) code.

If libutempter is available, mosh-server will record sessions in the
utmp file, which makes them visible to commands like who.

The file debian/control contains a list of the relevant Debian packages.

Usage

The mosh-client binary must be installed on the user's machine, and
the mosh-server binary on the remote host.

The user runs:

$ mosh [user@]host

A command may also be specified, for example:

$ mosh host -- screen -r

If the mosh-client or mosh-server binaries are installed outside the
user's PATH, mosh accepts the arguments --client=PATH and
--server=PATH to select alternate locations. More options are
documented in the mosh(1) manual page.

Mosh supports 256-color mode as long as the user's own terminal
does. Generally this means the TERM environment variable must be
set to xterm-256color or screen-256color-bce before running
mosh.

How it works

The mosh program will SSH to user@host to establish the connection.
SSH may prompt the user for a password or use public-key
authentication to log in.

From this point, mosh runs the mosh-server process (as the user)
on the server machine. The server process listens on a high UDP port
and sends its port number and an AES-128 secret key back to the
client over SSH. The SSH connection is then shut down and the
terminal session begins over UDP.

If the client changes IP addresses, the server will begin sending
to the client on the new IP address within a few seconds.

To function, Mosh requires UDP datagrams to be passed between client
and server. By default, mosh uses a port number between 60000 and
61000, but the user can select a particular port with the -p option.

Advice to distributors

A note on compiler flags: Mosh is security-sensitive code. When making
automated builds for a binary package, we recommend passing the option
--enable-compile-warnings=error to ./configure. On GNU/Linux with
g++ or clang++, the package should compile cleanly with
-Werror. Please report a bug if it doesn't.

Mosh ships with a default optimization setting of -O2. Some
distributors have asked about changing this to -Os (which causes a
compiler to prefer space optimizations to time optimizations). We have
benchmarked with the included src/examples/benchmark program to test
this. The results are that -O2 is 40% faster than -Os with g++ 4.6
on GNU/Linux, and 16% faster than -Os with clang++ 3.1 on Mac OS
X. In both cases, -Os did produce a smaller binary (by up to 40%,
saving almost 200 kilobytes on disk). While Mosh is not especially CPU
intensive and mostly sits idle when the user is not typing, we think
the results suggest that -O2 (the default) is preferable.